Three groups of persons vaccinated against rubella are being followed to determine the persistence of immunity after HPV77DE5, Cendehill, and RA 27/3 vaccines. Sera collected 3, 5, and 9-10 years following vaccination are being evaluated for HI and neutralizing antibodies, and the results compared with sera from natural immunes observed over the same period of time. Approximately 25% of the HPV77 vaccinees whose original antibody responses were minimal have lost detectable HI and neutralizing antibody over a 3-5 year period. Antibodies induced by RA 27/3 are longer lasting and no child has yet become antibody negative. Laboratory investigations have revealed that neutralization test results differ markedly depending on the source of virus used in the test. Thus virus grown in rabbit cells lacks hemagglutinin, and as a consequence HI antibodies do not participate in the neutralization of rabbit cell-grown virus; the result is a lack of correlation of HI and neutralizing antibody responses. The use of virus grown in rabbit cells in neutralization tests discriminates between antibody responses to HPV77 and to natural, wild virus infection: there is a marked lag in appearance of NT antibodies in HPV vaccinees, virtually all of the 2 month post-vaccinal sera being negative. This lag was not found when virus grown in monkey kidney cells was used. Characterization of rabbit cell grown virus indicated that the modification was phenotypic in nature.